Asphalt-based roofing systems and products are well known. They include, for example, asphalt shingles and asphalt roll roofing. Many conventional materials are utilized as raw materials in the manufacture of these asphalt roofing materials.
Asphalt roofing materials comprise a substrate which is filled and coated with various asphalt materials. Generally, the substrate is filled with a "saturant" asphalt to provide waterproofing and saturation of the substrate. The saturated substrate is usually sealed by application of a harder, more viscous "coating" asphalt to one or both sides of the substrate.
The exterior, outer, or exposed surface of asphalt roofing materials is generally provided with a covering of granular material or roofing granules embedded within the coating asphalt. The roofing granules generally protect the underlying asphalt coating from damage due to exposure to light, particularly ultraviolet (UV) light, and also improve both fire resistance and weathering characteristics. In addition, roofing granules are typically coated with a colored coating for aesthetic reasons. The roofing materials may include solid colors or blends of more than one color of granule, and specific colors of granule blends may be obtained by varying the proportions of different colored roofing granules combined in the granule blend. In addition, roofing granules with specific functional characteristics may be dispersed throughout the granular blend to achieve some desired performance of the roofing material. For example, algae-inhibiting roofing granules may be dispersed throughout a granular blend to inhibit the growth of algae on the roofing granules or roofing material.
In the roofing industry, it is desirable for the various sizes, types, or colors of roofing granules in a granule blend to be evenly dispersed across the surface of the roofing material to achieve a uniform appearance or functionality. However, even dispersion cannot be achieved if the roofing granules are inadequately blended or mixed before being applied to the roofing material. One common problem that occurs when various sizes, types, or colors of roofing granules are unevenly dispersed throughout a granule blend is referred to as granule segregation. When segregated roofing granules are applied to a roofing material that is later applied to a roof, the result is that some areas of the roof may vary in appearance or functionality.
One specific type of problem that arises when roofing granules are not properly blended is commonly known as "shading". Shading may be seen as the roof is viewed from different angles or under different light conditions, where those areas of a roof with segregated roofing granules may appear lighter or darker than other areas of the roof. This may happen even when the granule blend contains only roofing granules of a single color.
Shading is typically easier to perceive when viewing the roof at certain angles or under particular lighting conditions, but in fact may be visible any time the viewer is looking at the roof. Shading problems may occur on roofing materials where the granule blend includes various roofing granule colors, various roofing granule sizes, or both. For example, shading may occur on black shingles where different sized black roofing granules are segregated on the shingle, or similarly may occur in a mixture where the roofing granules are the same size but different colored roofing granules are more concentrated on one area of the shingle than others. Although the granule blend variations are often not detectable when looking at individual shingles or roofing materials, distinct lines or patterns of darker or lighter areas may appear when these roofing materials are actually applied to a roof. If the shading is distinct enough to detrimentally effect the appearance of the roof, the overall roof appearance may be considered unacceptable.
Attempts have been made to achieve better mixing of granule blends. For example, it is known to use some type of mixing, blending, or dispersing device that improves the roofing granule blending prior to placing the granule blend on the roofing material or shingles. Dispersing devices such as illustrated in FIGS. 6 and 7 have been used for this purpose in the roofing industry. More specifically, FIG. 6 illustrates a typical device for dispersing roofing granules. Radially oriented baffle plates are positioned within this device so that granules that enter the top of the device and fall through the device will be deflected by the baffle plates to spread the granules across the width of the bottom, or outlet, of the device which is typically wider than the top, or inlet, of the device. The dispersing device illustrated in FIG. 7 similarly may be used to disperse granules across a specific width; however, this device uses several angled baffle plates spaced from one another to split the streams of granules as they fall from the inlet toward the outlet of the device, thereby allowing for some mixing of the roofing granules as they fall through the device. Devices of this type typically operate most effectively when the granules flow freely through the device so that as granules strike the baffle plates, they can easily move about within the device. In other words, blenders of this type typically do not operate well in a "choke-fed" situation, where the device becomes completely filled with granules. Although these dispersing devices and blenders may improve roofing granule blending, they have not generally been able to achieve the level of dispersion necessary to eliminate granule segregation.
Another method used to achieve a more even dispersion of roofing granules is the use of dynamic mixers. Such mixers may also be used in combination with the dispersing devices described above. Unfortunately, certain groups of roofing granules tend to migrate in a pattern to certain areas of the granule blend during the mixing process so that a true uniform mixture won't necessarily be achieved. Although these patterns may sometimes be overcome by increasing the time and/or intensity of mixing, an undesirable side effect of increased mixing is that the frictional contact of the roofing granules with each other during mixing causes the roofing granules to wear down. Therefore, although increasing the time of mixing the roofing granules before applying them to the roofing materials may decrease roofing granule segregation and improve the uniformity of the granule blend, the roofing granules consequently may also become so excessively worn that they are no longer suitable for their intended application.
A like concern arises when dealing with granule blends where certain roofing granules are added to the granule blend in a specific proportion for a specific functional purpose. For example, when algae-inhibiting granules are mixed into a granular blend in the correct proportion, they help to preserve both the color and the life of the roofing material to which they are applied. If the algae-inhibiting granules are not evenly dispersed in the granule blend, the full benefit of these roofing granules may not be achieved. Again, although increasing the amount of roofing granule mixing would typically provide for a more uniform blend of the algae-inhibiting roofing granules within the granular blend, the roofing granules themselves may show excessive wear from the mixing process.
Clearly, beneficial results may be achieved by decreasing segregation and improving the dispersion of roofing granules in roofing systems and products while minimizing wear on the roofing granules. Even in situations where granule blends were generally considered satisfactory, improvements are still desirable.